A standardized mechanical interface (SMIF) has been proposed to reduce particle contamination by significantly reducing particle fluxes onto wafers. This end is accomplished by mechanically ensuring that during transport, storage and processing of the wafers, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient outside environment do not enter the immediate internal wafer environment.
Control of particulate contamination is imperative for cost effective, high-yielding and profitable manufacturing of VLSI circuits. Because design rules increasingly call for smaller and smaller lines and spaces, it is necessary to exert greater and greater control on the number of particles and to remove particles with smaller and smaller diameters.
Some contamination particles cause process defects, such an incomplete etching in spaces between lines leading to an unwanted electrical bridge. In addition to such physical process defects, other contamination particles may cause electrical failure due to induced ionization or trapping centers in gate dielectrics or junctions.
Modern processing equipment must be concerned with particle sizes which range from below 0.01 micrometers to above 200 micrometers. Particles with these sizes can be very damaging in semiconductor processing. Typical semiconductor processes today employ geometries which are 1 micrometer and under. Unwanted contamination particles which have geometries measuring greater than 0.1 micrometer substantially interfere with 1 micrometer geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries.
In typical processing environments today, "clean rooms" are established in which, through filtering and other techniques, attempts are made to remove particles having geometries of 0.03 micrometer and above. There is a need, however, to improve the processing environment. The conventional "clean room" cannot be maintained as particle free as desired. It is virtually impossible to maintain conventional clean rooms free of particles of a 0.01 micrometer size and below.
The main sources of particulate contamination are personnel, equipment, and chemicals. Particles given off by personnel are transmitted through the environment and through physical contact or migration onto the wafer surface. People, by shedding of skin flakes, for example, are a significant source of particles that are easily ionized and cause defects. Although clean room garments reduce particle emissions they do not fully contain the emissions. It has been found that as many as 6000 particles per minute are emitted into an adjacent one cubic foot of space by a fully suited operator.
To control contamination particles, the trend in the industry is to build more elaborate and expensive clean rooms with HEPA and ULPA recirculating air systems. Filter efficiencies of 99.999% and up to ten complete air exchanges per minute are required to obtain an acceptable level of cleanliness.
To minimize process defects, processing equipment manufacturers must prevent machine generated particles from reaching the wafers, and suppliers of gases and liquid chemicals must deliver cleaner products. Most important, a system must be designed that will effectively isolate wafers from particles during storage, transport and transfer into processing equipment. The Standard Mechanical Interface (SMIF) system has been proposed to achieve this goal. The SMIF concept is based on the realization that a small volume of still, particle-free air, with no internal source of particles, is the cleanest possible environment for wafers. Further details of one proposed system are described in the article "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING", by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115 and in the above cross-referenced applications.
The proposed SMIF system has three main components, namely, (1) minimum volume, dustproof containers are used for storing and transporting wafer cassettes; (2) canopies are placed over cassette ports of processing equipment so that the environments inside the containers and canopies become miniature clean spaces; (3) doors on the containers are designed to mate with doors on the interface ports on the equipment canopies and the two doors are opened simultaneously so that particles which may have been on the external door surfaces are trapped ("sandwiched") between the doors.
In the proposed SMIF system, a container is placed at the interface port on top of the canopy; latches release the container door and the canopy port door simultaneously. A mechanical elevator lowers the two doors, with the cassette riding on top, into the canopy covered space. A manipulator picks up the cassette and places it onto the cassette port/elevator or other location within the canopy of the equipment. After processing, the reverse operation takes place.
The SMIF system has been proved effective by experiments using prototype SMIF components both inside and outside a clean room. The SMIF configuration achieved a tenfold improvement over the conventional handling of open cassettes inside the clean room.
However, due to the space limitations within the canopy of the processing station, the size and configuration of the elevators and manipulators is important. Furthermore it is desirable that the equipment for removing the cassette holding articles to be processed from the standard mechanical interface container be confined to a small space when not in use while providing a long reach to adjacent equipment.